A new method of discovering new meteor showers from the IMO single-station video meteor database

Transcription

1 Science in China Series G: Physics, Mechanics & Astronomy 2008 SCIENCE IN CHINA PRESS Springer-Verlag phys.scichina.com A new method of discovering new meteor showers from the IMO single-station video meteor database LI Jun 1,2 & ZHU Jin 3 1 No.2 Middle School of Beijing, Beijing , China; 2 School of Journalism and Communication, Tsinghua University, Beijing , China; 3 Beijing Planterium, Beijing , China It has been found that unknown meteor showers could be efficiently discovered from the single station video meteor database of the International Meteor Organization (IMO) by assuming the geocentric velocity and adjusting it within the dynamically permitted range. The mean geocentric velocities of new meteor showers can be obtained, as well as the coordinates of the radiants. The activity period and maximum time can also be obtained if there are sufficient shower meteors. All single station video meteor observations between February 13 and 17 (from 2000 to 2005) in IMO s database are processed with this method. As a result, two new meteor showers, one near RA=245.10, Dec=41.82 in Hercules and the other near RA=233.03, Dec=17.04 in Serpenids, are discovered. Some dynamical characteristics of the new meteor showers are also determined. Considering the random nature on the selection of period in this work, it is expected that there are some more potential new meteor showers in IMO s video database. single-station video meteor database, the process of single-station video meteor database, discovery of new meteor showers The traditional methods for the determination of new meteor showers include plotting and double station techniques. Plotting is based on the backward prolongations of meteor paths. A meteor may belong to a shower only if its backward trace is through the nearby region of the corresponding radiant [1]. This is a direct method, which maintains rather high requirements for observers and high restrictions for further analysis. Double station observations record different positions of the same meteor on the star background, and then consequently compute the trajectory and orbit with geometry [2]. With this method, precise coordinates of the radiant can be determined. However, such observations of comparatively high requirements are relatively difficult to perform. Received June 23, 2007; accepted November 1, 2007 doi: /s Corresponding author ( xiaoluoluo88re@126.com; jinzhu@6jp.org.cn) Supported by the National Natural Science Foundation of China (Grant No ) Sci China Ser G-Phys Mech Astron Aug vol. 51 no

2 In the past decades, with the rapid development of the video technology, a huge amount of observational data have been collected, most of which are from the non-coordinated single station. Principally, most unknown meteor showers should have been recorded in the current observational data of video meteors. However, besides the orbit determination with double or multi-station observations [3-7], the utilization of single station observations is mostly limited to the investigation of the well-known meteor showers. While searching for the data of known meteor showers, it is noticed that IMO maintains a great many observational data from all over the world, in which most meteors are labeled as sporadic. Given them dynamical origin, the appearance of the sporadic meteors should be stochastic. Thus, it can be regarded as a steady flux and normalization standard on a large time scale. During the analysis of IMO s video meteor database with the data processing software RADIANT 1.43 [8], it is found that unknown meteor showers can be distinguished by manually adjusting the geocentric velocity within the dynamically permitted range. Beside the coordinates of radiants, rough geocentric velocities can be determined simultaneously. In the case of having adequate data, the activity period and maximum time can also be obtained. To evaluate this method, an arbitrary period is selected and two new meteor showers are successfully discovered, i.e. Herculids and Serpenids. 1 Method 1.1 Data All available video meteor data from 2000 to 2005 on the website of the IMO are downloaded. 1.2 Data reduction Data reduction is done with the meteor analysis software RADIANT 1.43, by Arlt. This routine is mainly used to analyze the radiants of the known meteor showers, especially those with known geocentric velocity. In our method, special treatment is applied to geocentric velocity, so that the software can be used to find new meteor showers, as follows: 1) Take a period of usually 3-5 d and then assume some central coordinates of the field of view, α center, δ center. According to α center and δ center, set the proper values for the geocentric velocity, minimum and maximum angular speed, daily motion in RA and solar longitude. While adjusting the above settings, it should be especially noticed that: The Exhaustive Attack method can be used for the test of geocentric velocity because the change of geocentric velocities is effective for the resultant coordinates of radiant. The experiments of several meteor showers indicate that the radiant is significantly changed in the computed distribution when the velocity variation is larger than 5 km/s. Thus practically, the step size of velocity attack is limited within 5 km/s. The radiant can appear as a highly concentrated region in the computed distribution only if the test velocity is close to the true value. Non-zero minimum angular speed is wanted. The lower the minimum angular speed in the analysis is, the more meteors can be included in the analysis. Nevertheless, slow meteors will be given too much weight in the computation. Indeed, slower meteors usually have slower measuring uncertainty, however, too much weight of very few meteors would affect the overall probability distribution of the radiant, which is unwanted in our work. The intention of our work is to statistically review the radiant distribution, not precisely. To avoid the serious error in the final result, the minimum angular speed is set as 7 /s in this work. LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

3 The maximum angular speed can be calculated with 1) 2 Vg π ω max = 2arctan sin θ, 2h 180 where V g is the geocentric velocity, h is the height of meteor, normally from 80 km to 100 km, and θ is the apparent angular elevation. This formula is applied to avoid the pollution from high speed meteors. 2) Selection of analysis method: There are three methods in RADIANT 1.43, i.e. tracings, intersections and probabilities. Probabilities is the commonly used one. The other two can be adopted to confirm the result. 3) Practically, there is always some interference from uninterested meteors in the same field of view, which may slightly affect the resultant distribution. Therefore, these meteors have to be excluded from the list of the analyzed meteors. The result of the retreatment after this process can be regarded as the pure one for our search. 4) Adjust the parameters of the new meteor showers according to the results to approach a more accurate velocity and radiant coordinates. According to the member meteors of the new meteor showers, we need to find an appropriate solar longitude to obtain the activity period and flux profile. 2 Example of new meteor shower discovery with the new method With the video meteor data between Feb. 13 and 17 from 2000 to 2005 in the IMO s video meteor database, the method proposed in sec. 1 is applied. 2.1 Determination of the Herculids (1) Enactment of parameters: Center coordination: α center = 240, δ center = 0 Dates: Feb 13th Feb 17th Geocentric velocity: V g = 35 km/s Minimum angular speed: 7 /s Maximum angular speed: 22 /s Daily motion: 1.0 Solar longitude: 325 It is found that 71 meteors belong to this shower. The geocentric velocity is 35 km/s. The coordinate of radiant is α = (245.10±0.5), δ = (41.82±0.5), as in Figure 1. (2) Activity profile of the Herculids. As there is no reliable record of the limiting magnitude and sky obscuration, the maximum cannot be found by computing the absolute flux. Thus, the sporadic meteors are adapted to the relative rate, i.e. defining Her% = N Her /N spo. Figure 2 shows the averaging of the relative rate over a range of solar longitude. A peak-like profile may occur between and But the observations in the database are insufficient to have further insight into this period. 1) Zhang M L, Meng H. The analysis of video meteor observations in 2004 November 17/18 and 19/20 in China LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

4 Figure 1 Radiant computed distribution of Herculids. Figure 2 Activity profile of Herculids. 1, The peak of Her% is located at solar longitude 327.2, with the strength of 47%. 2.2 Determination of Serpenids (1) Enactment of parameters: Center coordination: α center = 240, δ center = 0 Dates: Feb 13th Feb 17th Geocentric velocity: V g = 50 km/s Minimum angular speed: 7 /s Maximum angular speed: 32 /s Daily motion: 1.0 Solar longitude: 325 It is found that 72 meteors belong to this shower. The geocentric velocity is 50 km/s. The coordinate of radiant is α = (233.03±0.5), δ = (17.04±0.5), as in Figure 3. LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

5 Figure 3 Radiant computed distribution of Serpenids. (2) The activity profile of Serpenids. Like the case of the Herculids, only the normalization with sporadic background can be processed for this meteor shower based on the available data, namely setting Ser% = N Ser /N spo. The profile is shown in Figure 4. Figure 4 The change of the trend of Serpenids. 2, The peak value of Ser% is at solar longitude 326.2, with the peak strength of 86%. 3 Discussion and conclusions 3.1 Comparisons between new and traditional methods Method Advantages Disadvantages Plotting low requirements for equipments, directness low accuracy Double station orbit determination equipment restriction, observations design New method few requirements large amount of data necessary 1174 LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

6 3.2 Conclusions A new method for discovering new meteor showers from video meteor data is proposed. The application of this method on an arbitrarily five-day period results in the discovery of two new meteor showers, i.e. Herculids and Serpenids. The geocentric velocity, activity period and the maximum time are obtained. More observations are still needed for the confirmation of the two new meteor showers. Thus, the more accurate results can be obtained. The evaluation indicates that this method can be used to efficiently discover minor meteor showers and to determine some basic parameters if adequate data are available. There has been a huge amount of data in the database, and the observers all over the world are keeping contributing to it. It can be expected that much more unknown meteor showers will be recorded in the IMO s database and be discovered. The author would like to thank the Beijing Youth Science and Technology Club. Appendix Table A1 Meteor list of the Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids (To be continued on the next page) LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

7 (Continued) Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Herculids Table A2 Meteor list of the Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids (To be continued on the next page) 1176 LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

8 (Continued) Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids (To be continued on the next page) LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no

9 (Continued) Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Serpenids Belkobich O. Visual meteor observations What is the use? In: Roggemans P, Arlt R, eds. Proceedings of the International Meteor Conference. Potsdam: International Meteor Organization, Molau S, Nitschke M, de Lignie M, et al. Video observations of meteors: History, current status, and future prospects. WGN J Int Meteor Organ, 1997, 25: Arlt R, Rendtel J. The activity of the 2004 Geminid meteor shower from global visual observations. Mon Not Royal Astron Soc, 2006, 367(4): de Lignie M, Betlem H. A double-station video look on the October meteor Showers. WGN J Int Meteor Organ, 1999, 27: Shigeno Y, Shioi H, Tanaka S. Double-station TV meteor observations in WGN J Int Meteor Organ, 1997, 25: Shigeno Y, Shioi H. Double-station TV meteor observations. WGN J Int Meteor Organ, 1996, 24: Triglav-Cekada M, Arlt R. The summer pegasids from IMO video data. WGN J Int Meteor Organ, 2005, 33: Arlt R. The software RADIANT. WGN J Int Meteor Organ, 1992, 20: LI Jun et al. Sci China Ser G-Phys Mech Astron Aug vol. 51 no